Use guaranteed const for associated object key

[morganchen12]

From the Foundation docs for -[NSString UTF8String]:

This C string is a pointer to a structure inside the string object, which may have a lifetime shorter than the string object and will certainly not have a longer lifetime. Therefore, you should copy the C string if it needs to be stored outside of the memory context in which you use this property.

Since the inner pointer may have a shorter lifespan than the owning NSString, it shouldn't be used as a constant value. This PR replaces it with the address of the variable, which is guaranteed to be constant during the lifetime of the string constant.

[paulb777]

Nice find! Does it fix any specific failures?

We could roll this into a 7.12.1 along with a fix for #140

cc: @ncooke3

[ncooke3]

Also would be good to have a changelog entry. Doesn't need to be in this PR though.

[ncooke3]

lgtm. the ci failure is safe to ignore